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Glossary

Annular eclipse/annulus
The moon orbits the earth in an oblong orbit, moving nearer and farther from the earth and therefore appearing bigger when nearer and smaller when farther. An annular solar eclipse occurs when the entire moon moves in front of the sun but the moon is so distant that its disk does not cover the sun's disk completely, leaving a brilliant annulus or ring of sunlight (“annulus” is Latin for “ring”). In contrast, a partial solar eclipse occurs where only part of the moon covers just part of the sun, so the sun’s disk has the shape of a cookie with a bite taken out.

Broadband filter
Filter designed to improve the visibility of various deep sky objects by selectively reducing the transmission of unwanted wavelengths of light

Chromosphere
A reddish gaseous layer immediately above the photosphere of the sun. Together with the corona, it constitutes the sun's outer atmosphere which can be seen (without special instruments) only during a total solar eclipse.

Chronometer
An instrument for measuring time, especially one designed to keep accurate time on shipboard or in remote locations in spite of motion or variations in temperature, humidity, and air pressure.

Collodion
A solution of nitrocellulose in a mixture of alcohol and ether, used for coating film in a nineteenth-century photographic process.

Coronal mass ejections
A coronal mass ejection (CME) is a massive burst of gas and magnetic field arising from the solar corona and being released into the solar wind.

Coronal Streamers
A wisp-like stream of particles traveling through the sun's corona, visible in images taken with a coronagraph or during a total solar eclipse. Coronal streamers are thought to be associated with active regions and/or prominences and are most impressive near the maximum of the solar cycle. Although they can be longer than the diameter of the sun, they are very tenuous; the material in them gradually moves away from the sun and becomes part of the solar wind.

Gibbous
A phase of the Moon between First Quarter and Full Moon, or between Full Moon and Last Quarter, when the illuminated portion is greater than a semicircle and less than a full circle

Glycerine clock
A device to move or rotate a mechanical device at an accurately controllable speed. It usually consisted of a cylinder (fitted with a piston) filled with glycerine, which was allowed to flow out through adjustable valves so that the piston moved at a slow rate. This was used in astronomy in the 19th and early 20th centuries, for example, to turn a telescope slowly so that it tracked the motion of the sun across the sky, allowing a time exposure to be made during an eclipse.​

Gravitational deflection of light
The deflection of a beam of light that passes close to a massive object (such as the sun), which was predicted by Einstein’s General Theory of Relativity. This effect was first detected by observing the slight change in the apparent position of background stars close to the sun that became visible during the total solar eclipse of 1919.

Hydrogen-alpha filter
An optical filter that passes only a narrow band of red light. By blocking most of the light from the sun, its use allows for safe observation of the entire solar disc, providing views of prominences, chromosphere, and surface details such as sunspots, flares, filaments, and granulation.

Periodicity of Sunspots (sunspot cycle)
The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number of sunspots, flares, and other manifestations).

Photo heliograph
A specialized telescope adapted to taking photographs of the sun.

Pneumatic commutator
A device using air pressure for operating automatically a number of instruments, such as cameras, in a prescribed sequence and for prescribed exposure times.

Polar axis
The axis of an equatorially mounted telescope that is at right angles to the declination axis and parallel to the earth's axis of rotation, about which the telescope is turned to follow the apparent movement of celestial objects resulting from the earth's rotation.

Prominence
Also called solar prominence. An eruption of a bright red flame-like tongue of relatively cool, high-density gas from the solar chromosphere into the corona where it can be seen during a solar eclipse or by observing strong spectral lines in its emission spectrum.

Solar corona
A corona (Latin, 'crown') is a white or silvery aura of plasma that surrounds the sun; the outer atmosphere of the sun, normally visible only during a total eclipse.​

Solar limb
The “edge” of the sun’s disk

Solar prominence
Also called prominence. An eruption of a bright red flame-like tongue of relatively cool, high-density gas from the solar chromosphere into the corona where it can be seen during a solar eclipse or by observing strong spectral lines in its emission spectrum.

Solar radius
The radius of the sun.

Spectrograph
An apparatus for observing the spectrum of light from an incandescent body (such as the sun) by separating the light into its constituent colors. Used to determine the chemical composition of such a body.

Sunspot cycle
The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number of sunspots, flares, and other manifestations).

Transit
A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point. The most notable transits are the rare ones of Venus and Mercury when they are observed to cross the sun’s disk.

Total solar eclipse
Eclipse of the sun in which the moon completely obscures the solar surface and thereby cuts off all direct rays of sunlight from the observer

Totality
The brief time (from a few seconds up to a few minutes) when the sun is completely obscured by the moon during a solar eclipse.

Wet collodion process
The wet plate collodion process, developed in the 1850s, differs from the previous photographic methods because rather than using paper, the light-sensitive materials are placed on glass. Prior to exposure, the glass is coated with a mixture of bromide, iodide, and chloride which is dissolved in collodion. This solution is applied to the glass and allowed to gel. While it is still wet, the plate is placed inside the camera and exposed to light. As it dries, the solution is no longer sensitive to light and a permanent image is created. Because the negative is on glass, it is on a more stable surface and the images can be more detailed than those caught on paper.